Final answer:
Charles Darwin developed the theory of evolution by natural selection in the 19th century, which is true. His work preceded the discovery of genes as the units of heredity, laying the groundwork for modern evolutionary biology.
Step-by-step explanation:
Indeed, it is true that Charles Darwin developed the theory of evolution by natural selection in the 19th century before the identification of genes as units of heredity. Darwin's seminal work, On the Origin of Species, published in 1859, proposed the mechanism of natural selection as the means by which species evolve over time. While Darwin did not know about DNA or the concept of genes as we understand them today, his theory set forth the foundation for our current understanding of evolution and heredity, which would be greatly enhanced by the later discovery of genetics.